Papermaking machine



July 9, 1963 R. w. MARTINEK 3, 07

PAPERMAKING MACHINE Filed Oct. 12. 1960 United States Patent l 3,097,107 PAPERM AKENG MAQHLNE Richard W. Martineir, Appleton, Wis., assignor to Kimberly-Clark Corporation, Neenah, Wis., a corporation of Delaware Filed Oct. 12, 1969, Ser. No. 62,258 8 Claims. (Cl. 117102) This invention relates to the production of coated paper webs and the like, and is particularly directed to methods and apparatus for the attainment of coated paper webs of improved surface and printing characteristics.

This application is a continuation-in-part of my c0- pending application, Serial No. 861,884, filed December 21, 1959, now abandoned and assigned to the same assignee as the present application.

The invention particularly contemplates the production of coated paper webs which find utility for book" and publication grade papers of high quality. Such papers are commonly prepared by applying an aqueous dispersion of a mineral coating to the paper web; the web itself may be of a light or heavy basis weight and the web is subjected to tension on the application of the coating material and in the passage of the web through the coating apparatus. Since the paper web is wetted by the coating composition, the combination of factors involved in the coating operation frequently results in some web expansion during the process, the web expansion generally being greater for webs of lighter basis weight. This web expansion is objectionable, particularly in coating processes involving blading operations of the coating Zllfter application of coating to the web, for the expansion on some occasions tends to develop a bubble or bagginess in the web prior to the blade and the web then may break in the blade nip.

The methods and apparatus of this invention are adapted for coating of both the light and heavy weight base papers and for operation either on the paper making machine or olf of the machine as in conventional coatcrs. Moreover, the methods and apparatus of the invention are particularly applicable in the coating of wide webs at high speed and in the coating of dry webs or webs having a degree of moisture which may be up to 73% by weight of the web when the web is presented to the coating nip of the apparatus.

The method of invention involves applying at an applicator roll-backing roll nip an excess of the coating material and subsequently blading ed the excess coating material while minimizing the effects of wetting of the web and of web tension. Accomplishment of this is achieved generally by directing the web through the nip of the rolls and a doctor blade nip sequentially in such manner that detrimental tension is not applied to the web. The backing roll and the applicator roll which form a first pressure nip are suitably individually driven. The pressure at this first nip is preferably low, and is generally considerably lower than in conventional roll coating operations. The web of paper is drawn through the first nip and the coating material is applied to the web, the coating being in the form of a thin liquid film on the applicator roll.

The paper Web is then drawn from the first pressure nip for a short length over the periphery of the applicator roll and away from the surface of the backing roll. This increases the contact of the web with the film of coating material on the applicator roll. Also, the web is itself then well supported by the applicator roll, and fluttering of the web is inhibited.

The web with the coating thereon is so directed, circuitously from the surface of the applicator roll that it approaches the backing roll at the backing roll-doctor 3,097,107 Ce Patented July 9, 1963 blade pressure nip very nearly tangential] y. For this purpose a web carrying nip-out roll is provided. This nipout roll is positioned between the two pressure nips out of contact with the backing and applicator rolls and is itself suitably freely rotatable. Also, this nip-out roll has a peripheral surface which lies substantially on a tangent to the backing roll at the nip between the backing roll and blade. Accordingly, the web may be drawn from the applicator roll over the web carrying nip-out roll and fed to the backing roll adjacent and immediately ahead of the blade.

A minimum of contact, but a positive contact of the web with the backing roll is suitably achieved prior to the entry of the web to the blade nip; this provides for firm support of the web in the nip. Further, a minimum of contact but also a positive contact of the web is maintained with the backing roll subsequent to the nip. Limiting the contact of the web with the backing roll adjacent the blade nip permits tension applied on the web subsequent to the blade to cause slippage of the web on the backing roll drawing out any bubbles 01' bags which may tend to occur in the web. Thus, while the speed of the Web at the blade hip is very nearly that of the backing roll, some slight slippage under the blade may occur to provide for takeup of any slack.

Suitably, the web, prior to entry to the applicator roll-backing roll nip, is directed to the surface of the backing roll, that is, it is prewrapped. The web thus passes from the backing roll through the applicator nip and across a surface portion of the applicator roll to the nip-out roll and then to the blade nip in a winding or sinuous path. The web is thus kept taut through the coating apparatus but the tension is insufficient to cause web breakage even on the lighter basis webs.

The process of invention is applicable to webs customarily employed in the production of high quality printing paper for use on any of the conventional type printing presses, for example, rotogravure, offset, letter press, and flexographic.

Conventional mineral coating formulations are employed in the practice of the invention. Such compositions generally include mineral pigments such as clay, calcium carbonate, titanium dioxide, satin white, and the like, and an adhesive such as starch. The viscosities and solids contents of the coating compositions may vary widely, also as is well known. Coating solids, for example, may range from 40 to 62% by weight on the composition, while viscosities as measured by a Brookfield viscometer, using spindle No. 6 at rpm. and ternperatures of 25 to 60 C. may range from 15 to poises.

It is therefore an object of the invention to provide an improved coating apparatus and method which consistently produces a fine bladed coating surface on light basis weight webs, particularly those that are very wide and have appreciable non-uniformities in porosity, finish, moisture, bulk, and basis weight across the web width.

In particular, it is an object of the invention to provide an improved blade coating apparatus which allows the web to be drawn underneath the coating blade at a faster speed than a backing roll under the blade for the purpose of taking up the expansion of the web that occurs subsequent to the application of coating to the web in a nip between the backing roll and a coating applicator roll.

The invention consists of the novel constructions, arrangements, devices and methods to be hereinafter described and claimed for carrying out the above stated objects and such other objects as will be apparent from the following description of a preferred form of the invention, illustrated with reference to the accompanying drawings wherein:

FIG. 1 is a diagrammatic elevational view of coating apparatus embodying the principles of the invention and including a backing roll having a pressure nip with a coating applicator roll through which the web to be coated passes and a blade having a nip with the backing roll through which the web also passes;

FIG. 2 is a fragmentary side view on an enlarged scale of the backing roll in the vicinity of its nip with the blade and the web passing through this nip; and

FIG. 3 is a view similar to FIG. 2 on a still further enlarged scale.

Like characters of reference designate like parts in the several views.

The illustrated coating mechanism may be seen to comprise a pair of counter-rotating metering rolls and 11 which are disposed on horizontal axes in the same horizontal plane with their surfaces in contact for maintaining a pool of coating 12 between the rolls. The coating may be supplied between the rolls from any suitable coating source (not shown).

A coating applicator roll 13 is provided in contact with the roll 11. The roll 13 is substantially larger in diameter than the roll 11 and has its center disposed on the same horizontal plane as the rolls 10 and 11. A backing roll 14 of substantially the same size as the roll 13 is in contact with the roll 13. The centers of the two rolls 13 and 14 extend parallel with each other, and the roll 14 is preferably positioned off to one side of the roll 13, a line 15 passing through the centers of the two rolls 13 and 14 extending at an angle A with respect to horizontal. The angle A in a certain embodiment of the apparatus was 60 degrees, although it will be understood that this angle may be varied. Each of the rolls 13 and 14 is surfaced with a rather stiif rubber-like resilient covering 16.

Paper web 17 may be passed between the rolls 13 and 14 for coating the web, as will be hereinafter described. The web may be supplied from the drier section of a conventional paper machine, assuming continuous onthe-machine operation of the coating apparatus, but for convenience of illustration, the web is shown fed from a supply roll 18 as during off-the-machine operation. The roll 18 is maintained under frictional drag or back tension, the optimum value of which may vary widely for different runs but which is predetermined and fixed for each set of operating conditions by a conventional type of adjustable tension device 19 that may be associated with a drier roll for on-the-machine operation. The web 17 may be supplied to the rolls 13 and 14 by any suitable guiding devices, such as guide rolls 20 and 21. The guide roll 20 which is adapted to contact the top surface of the web 17 may be a conventional Mount Hope roll for spreading the web slightly to maintain it free of wrinkles, and the roll 21, which contacts the lower surface of the web 17, may be of the usual solid type.

The take up mechanism for the web may comprise a driven wind-up roll 22 to which the web travels from a drier roll 23. The web may travel directly from the backing roll 14 to the drier roll 23 or, if desired, suitable guide rolls (not shown) for the web may be positioned between the backing roll 14 and the drier roll 23.

The coating apparatus comprises a so-called nip-out roll 24 which is located to one side of and between the rolls 13 and 14, and the apparatus also comprises a coating blade 25, which is suitably mounted by means of a blade retainer 26. The blade 25 forms a nip with the roll 14 and is provided with a honed edge 25a which is held against the roll 14 with a force sufficient to flex the blade. The blade holder 26 is positioned at an angle a, such as, for example, 50, to a tangent to the roll 14, and the blade 25 may be flexed to extend at an angle of 35 to the tangent, for example, at the blade end. The blade 25 may be of any suitable spring material, such as spring steel.

The web 17 extends over the periphery of the roll 14 for an angle b before entering the nip between the rolls- 13 and 14. The web passes through the nip of the rolls 13 and 14 and extends over the periphery of the roll 13 for a small distance c in passing on to the nip-out roll 24. In order to provide the are 15 of web travel on the roll 14, a peripheral web carrying surface of the roll 21 is disposed above the tangent 15a at the nip of the rolls 13 and 14, which tangent is perpendicular to the center line 15. Similarly, a peripheral web carrying surface of the nip-out roll 24 is disposed below the tangent 15a, and between the tangent 15a and the adjacent peripheral surface of the applicator roll 13, so that the web 17 remains on the surface of the roll 13 for the distance 0 subsequent to the nip between the rolls 13 and 14. The web passes around the nip-out roll and thence through the nip between the roll 14 and the blade 25. The nip-out roll 24 has a peripheral surface substantially on the tangent to the roll 14 at the blade 25, and the same is true with respect to a peripheral surface of the drier roll 23, so that the web lies substantially on this tangent and is in substantially a straight line between the nip-out roll 24 and the drier roll 14 and has only a small peripheral contact with the backing roll 14 for distances d and 2 prior to the blade 25 and trailing the blade 25. It is thus apparent that the roll 24 is located opposite to and holds the web 17 out of contact with the peripheral surface of the roll 14 between the nip of rolls 13 and 14 and the nip of roll 14 and blade 25.

The rolls 10, 11, 13, 14, 20, 21, 22, 23, and 24 and the blade 25 are maintained in fixed relation with respect to each other. being mounted in suitable side frames of the machine (not shown). As many of the rolls may be driven as necessary for passing the web through the coating apparatus olf of the supply roll 18 and onto the wind up roll 22. In particular, the rolls 13 and 14 are separately driven, with the roll 13 being driven at a lower speed than the roll 14, as will be hereinafter more fully described. The blade 25 is maintained in forceful contact with the roll 14 and the Web 17 passing over the roll 14 by any suitable force applying mechanism (not shown).

In operation, the paper web 17 is drawn from the supply roll 18 over the Mount Hope roll 20 and the web supporting roll 21 into the nip between the applicator roll 13 and the backing roll 14. The Mount Hope roll 20 functions to maintain the web spread uniformly, and the roll 21 directs the web properly into the nip between the rolls 13 and 14 so that the web maintains contact with the backing roll 14 for the angle b before actually passing into the nip between the rolls 13 and 14. The web is maintained under tension particularly between the roll 21 and the nip between the rolls 13 and 14 by means of the tensioning device 19.

The rolls 13 and 14, being driven, draw the web through the nip between the rolls 13 and 14. The metering rolls 10 and 1] rotate along with the roll 13, and a pond of. coating 12 is maintained between the rolls l0 and 11 from any suitable coating source. The roll 11, as it rotates, carries coating on its surface to the surface of the applicator roll 13 so as to apply coating on the surface of the web 17 as the web 17 passes through the nip between the rolls 13 and 14. Both of the rolls 13 and 14 have a resilient covering as previously described, and the rolls 13 and 14 thus deform to some extent at the nip between the rolls and squeeze the web at the nip, so that only a predetermined amount of coating is applied on to the web as it passes through the nip.

The web passes from the nip between the rolls 13 and 14 over a small portion 6 of the periphery of the roll 13 on to the nip-out roll 24, and from thence the web passes substantially tangentially over the backing roll 14 and under the blade 25 on to the drier roll 23, having an osculating contact with the backing roll 1.4. The blade 25 functions to blade off excess coating on the web, so that the excess coating drips downwardly along the blade, and to smooth the surface of the coating so as to provide the web with an extremely smooth coated surface. The

web is drawn over the drier 23 and any other driers that may be necessary to dry the web, and the web is then wound on to the wind up roll 22.

As many of the rolls 1%, 11, 13, 14, 20, 21, 22, 23, and 24 are driven from sources of power to maintain a tension in the web, which is sufficiently low to asSure that the Web as it passes from the supply roll 17 to the wind up roll 21 will not break. Just enough tension is preferably kept on the web 17 to maintain it sufliciently taut that wrinkles do not form as the web passes through the coating apparatus. Obviously, too great a tension in the web will cause breakage, and the more slack that the web is, the less is the web breakage that can be expected. The tension just prior to the nip between the rolls 1.3 and 14- is, however, preferably higher than the tension in the web subsequent to the blade 25. The Web tension just prior to the nip between the rolls 13 and 14 may amount to between 3 to 5 pounds per lineal inch of the web, and the tension just subsequent to the blade 25 may amount to approximately 0.2 pound per lineal inch of the web, for example, and may vary from .15 pound per lineal inch to 1 pound per lineal inch for satisfactory operation.

The paper carrying roll 21 in the illustrated embodiment is so positioned that the web 17 passes over the exterior surface of the roll 14 for a certain arc b before entering the nip between the rolls 13 and 14. This are may be on the order of 90 degrees, but a much smaller arc is satisfactory, for example, an arc of 15 to degrees. Instead of passing the web 17 on to the surface of the roll 14 before the web enters the nip between the rolls 13 and 14,

alternately the web may be passed on to the surface of the lower roll 13. The are of travel of the web on the lower roll 13 also is not critical. It is, however, preferable that the web 17 should not enter directly into the nip between the rolls 13 and 14, since such passage of the web may cause fluttering of the web as it enters the nip with resultant coating defects. Also, there is a desirable smoothing action of either of the top roll 14 or the lower roll 13 in removing wrinkles out of the web before the web enters the roll nip. The rolls 13 and 14 in a particular embodiment were 36 inches in diameter, but it is, of course, obvious that rolls 13 and 14 of other sizes may equally as well be used.

The rolls 13 and 14 are preferably offset with respect to each other, such as at an angle A of 60 degrees. This offset is provided in order that a nip-out roll 24 of sulficient diameter for insuring against roll whip may easily be provided; however, it will be obvious that the rolls 13 and 14 may be positioned at some other angle A with respect to each other or may even have their axes vertically disposed with respect to each other while still providing for a nip-out roll 24.

The nip-out roll 24 is located so as to draw the web along a peripheral portion of the applicator roll 13 for a substantial distance c, out of contact with the surface of the backing roll 14. On an applicator roll 36 inches in diameter, the distance 0 of contact of the web 17 on the surface of the roll 13 may, for example, be 10 inches but the distance c is preferably less than this amount and may be only 3 to 5 inches, for example. It is desirable to have the web in contact with the lower roll 13 for this small distance 0, so that the web is not drawn straight out of the nip between the rolls I3 and 14; inasmuch as the web, if drawn straight out of the nip, will tend to whip up and down due to the tackiness of the coating on the lower surface of the web and its tendency to stick to the bottom roll 13 particularly since the coating is starting to set after it leaves the nip. The coating should not be allowed to dry unduly before the web is stripped off the lower roll 13, and it is thus undesirable to keep the sheet on the lower roll 13 any farther than necessary, but a distance of 10 inches has proved not to be excessive.

There is preferably a differential in speed between the applicator roll 13 and the backing roll 14, the applicator roll moving at a slightly slower speed, such as 10 to 50 feet per minute or in the order of 1% slower, than the top roll 14. Thus in the embodiment illustrated, applicatovr roll 13 may have a diameter of 36" and a surface speed of 898 f.p.m., while backing roll 14 also has a diameter of 36" and rotates at a surface speed of 910 f.p.m. Both of the rolls 13 and 14 are driven from suitable power sources, and the roll 14 has a peripheral speed substantially the same as the speed of the web 17. The roll 13, in being held to a lower peripheral speed than than of the roll 14, aids withdrawal of the web 17 from the roll 13. There is a tendency for the web 17 to follow the lower roll due to the tackiness of the coating on the lower roll and on the lower surface of the web 17 after the nip between the rolls 13 and 14; suificient tension is maintained on the web 17 after the nip to pull the web off the lower roll 13, with the lower roll speed aiding web withdrawal, so that the web does not travel down the peripheral surface of the roll 13 an undue distance. The lower roll 13 thus acts as a brake on the web, effective through the coating, a portion of which follows the roll itself, con tact with the coating tending to keep the web taut between the nip of the rolls 13 and 14 and the nip of the blade 25 on the roll 14, preventing wrinkles in the web across the web width between these two hips and giving a better control of the main nip between the rolls 13 and 14.

A minimum amount of wrap of the web on the top roll 14 is desirable both before and after the blade 25. A wrap of approximately 1 to 3 inches both before and after the blade 25 (distances d and a) has been found satisfactory. The tension on the web 17 beyond the blade 25 draws the web out from underneath the blade 25 at a very slightly greater speed than the peripheral speed of the roll 14, and the greater that the area of contact is of the web 17 underneath the blade on the roll 14, the greater is the tendency for the sheet to stick and lock to the surface of the roll 14 in the vicinity of the blade 25 and resist the effect of the tension on the sheet beyond the blade 25 to pull the web between the blade and the roll 14, particularly since the surface of the roll 14 is resilient. The nip-out roll 24, the roll 23, the backing roll 14, and the blade 25 are preferably so located with respect to each other that the portion of the web extending between the rolls 24 and 23 lies in substantially a straight line, the blade end which holds the web against the roll 14 depressing the web out of straight line configuration only slightly.

The tension in the web above the blade 25 pulling the web between the blade 25 and the roll 14, with the web slipping over the resilient surface of the roll 14, takes up the slack that would otherwise occur in the web between the nip of the rolls 13 and 14 and the nip of the blade 25 on the roll 14 due to expansion of the web caused by the application of the liquid coating to the web by the applicator roll 13. The tension in the web 17 at any point on the top coating roll 14 in the vicinity of the blade 25, due to the tension in the web just below the drier roll 23 and the locking effect of the web on the resilient roll surface, decreases with the distance the web is in contact with the roll 14 measured from the point at which the web leaves the roll 14. Therefore, the distance the web is maintained on the surface of the roll 14 in the vicinity of the blade 25 is held to a minimum, the web extending practically at a tangent to the surface of the roll 14 and having an escalating contact with the roll surface, so that there is a substantial pull on the web 17 between the nip of the rolls and the rolllade nip.

The blade 25 is preferably flexible-in one particular example it was formed of hardened flexible steel .012 inch in thickness, and the blade itself was 3 inches long, but 2% inches was embraced by the blade holder 26. The blade was applied on to the roll 14 with a pressure of 1 to 3 pounds per lineal inch. The holder 26 extended at an angle a of 50 degrees with respect to a tangent on the backing roll 14, and the blade flexed with a 15 degrees 7 defiection in the inch length of the blade between its holder and its end in contact with the web. Of course, the blade angles and dimensions of blade may be varied, but the above dimensions have proved satisfactory for preventing coat scratching with a coating of a conventional weight.

The angle from the center of the roll 14 that i included between the nip of the rolls 13 and 14 and the point of application of the blade may be varied while obtaining satisfactory operation of the coating apparatus, and the blade 25 may be located at other positions about the roll 14. Preferably, however, the dwell between the nip of the rolls 13 and 14 and the nip of the blade 25 on the backing roll 14 is minimized, so that the coating does n dry unduly between these two nips, rendering it difficult for the blade 25 to blade off excess coating and causing scratches if the coating is too hard at the time the blade is elicctive on the coating. In the particular embodiment described wherein rolls 13 and 14 were 36" in diameter and driven at the specific speeds noted hereinbefore, the blade 25 Was on an approximately 3 oclock position and the nip was approximately on the 7 oclock position for the right hand coater which is the type illustrated. For a reversed coater, of the left hand type, the nip would be at the 4:30 o'clock position and the blade would be at the 9 oclock position. An included angle of 135 degrees would thus exist between the nip and the blade in each case. An included angle between 90 and 180 is considered particularly suitable.

The diameter of the nip-out roll 24 is not critical, but it should be structurally stiff enough so that it does not whip at the web speeds that are used. The roll 24 in the particular embodiment noted above and illustrated in FIG. 1 had a diameter of approximately 12 inches. The distance 0 that the web follows the bottom roll 13 is determined principally by the sizes of the rolls 13, 14, and 24- and their geometrical location with respect to each other. The roll sizes and relative locations should preferably be such that the above described conditions exist, namely, the dwell of the web 17 between the nip of the rolls and the nip of the blade 25 on the roll 14 should be a minimum; the web should contact the roll 13 for only a short distance after the nip between the rolls 13 an 14; and there should be only a short distance of contact of the web on the roll 14 before and after the blade 25.

With the web 17 contacting the roll 13 subsequent to the nip between the rolls and contacting the roll 14 adjacent the nip between the blade 25 and roll 14, there is substantially independent control of the two nips. The web is thereby maintained taut between the two nips, and exercise over the action of the nip between the rolls may be had by varying the speed of the roll 13 and the pressure of the nip, while control over the blade nip may be had by varying the blade pressure and the tension on the web subsequent to the blade.

In order that the coating applied by the applicator roll 13 may be as uniform as possible, there is a substantial pressure maintained between the rolls 13 and 14 cansing some deformation of the resilient coverings on these rolls. The pressure of the nip between the two rolls may be varied considerably while yet maintaining uniform coat application. The pressure at the nip may be varied between 2 and 15 pounds per lineal inch, for example. Too much nip pressure leads to wrinkling of the web, and too little pressure tends to provide non-uniform coverage of coating; and a compromise nip pressure thus is used, suflicient to give good coat coverage. The distance c that the web wraps around the applicator roll 13 also has a bearing on the amount of coating that is taken by the web 17, the distance c being increased in order to increase the coating applied to the web. The pressures between the metering rolls 10 and 11 and between the rolls 1]. and 13 also have a bearing on the coating applied, and these pressures are maintained sui'ficiently low and the distance c sufficiently great that enough coating is applied to the web 17 for assuring that there will be a substantial drip or flow from the blade 25 as it blades off the excess coating from the web. This flow on the blade tends to clean the blade; and, furthermore, if there is an excess of coating bladed off, a uniform distribution of coat is assured. The angle of the blade 25 with respect to the roll and the pressure of the blade may also be varied. If the angle a is reduced, there tends to be more coating scratches; and, on the other hand, if the angle a is increased, the coat weight is reduced. Thus, the angle at which the blade is disposed is also a compromise depending on the coat weight desired as well as the qualities of the web that is being coated. The coat weight is controlled by the pressure applied to the blade, the coat weight decreasing with increased blade pressure.

In view of the fact that the surfaces of the rolls 13 and 14 are of rubber-like material, these rolls tend to wear. The various rolls of the coating apparatus are preferably adjustable with respect to each other, so that when the radius of the roll .14 is reduced by wear, the web 17 is ncverthclcss maintained travelling substantially as a tangent across the surface of the roll 14; and, of course, the rolls 13 and 14 are maintained together under pressure so as to provide the coat applying nip between the two rolls.

Although the distances d and e that the web contacts the upper roll 14 both before and after the blade 25 are small, it is desirable that there be at least some travel of the web on the roll 14 both before and after the blade 25. This is due to the fact that the upper roll 14 tends to smooth out any wrinkles that might occur in the web 17 prior to the coating being bladed off by the blade 25; and, if the web were pulled directly out of the nip between the blade 25 and the roll 14, web defects would thereby be aggravated.

My improved method of coating and apparatus therefor, as described, advantageously prevent creasing f the web as it passes beneath the blade 25 by preventing the bagging or bubble that has occurred in prior blade coaters. A small area of contact of the web 17 on the backing roll 14 is maintained adjacent the blade 25, assuring that the tension on the web above the blade is effective to pull the web through the nip of the blade to take up any excess web that may occur below the blade due to web expansion or other causes. The nip-out roll 24 holds the web out of contact with the backing roll 14 between the two nips allowing the tension to be thus effective and allowing a light basis weight web to be coated without utilizing an unduly high tension that might cause breakage of the low basis weight web. The coating apparatus is very effective, regardless of the fact that the web may be formed from low quality furnish tending to provide relatively non-uniform finish and other qualities across the deckle and even through the web is extremely wide, approaching a width of 200 inches, for example.

Although this advantageous operation of the coating apparatus is particularly suitable for light basis weight webs, it will be apparent that heavier basis weight webs may also be satisfactorily coated with this apparatus. There are, of course, other conditions which may be controlled in coating, such as moisture content of the web, coating viscosity and the solids content of the coating. It has been determined that these factors are not critical with respect to the coating apparatus of the invention, and these qualities of the web and coating may be varied within conventional limits while still obtaining the satisfactory operation of the coating apparatus. The machine speed also is not critical with the coating apparatus. The angles and arcs of wrap of the web need not be changed with increased machine speeds; however, it may be expected that, with increased speeds, there may be less coating strike-in, lessening the degree of web bagging that might occur, due to the shorter time that the web is subjected to the coating as it passes through the coating apparatus.

It is, of course, understood that two such coaters as above described may be utilized for coating the two sides of a single web. In this case, the backing roll of the first coater is driven at the same speed as the speed of the web entering the first ooater; and the backing roll 14 of the second coater is driven at the same speed as the web enters the second coater which is greater than the speed of the backing roll in the first coater by a small amount that is equal to the expansion of the sheet due to the application of coating thereto.

As noted hereinbefore, paper webs customarily employed in the production of high quality printing papers are utilized in the practise of the invention. Exemplary of such are all-sulphite sheets and sheets containing 35% by weight of sulphite and 65% by weight of groundwood. In general, sheets containing groundwood are somewhat more absorptive than the ali-sulphite sheets, as is well known in the art. Similarly, sheets exhibiting high porosity and roughness tend to be more absorptive and accordingly to expand more readily. However, utilizing the winding path over the nip-out roll as described permits ready passage of such webs or sheets without undue tension.

It should be noted that while the designations of light and heavy basis weight are somewhat arbitrary, it may be understood for the purpose of this invention that heavy basis weight webs are those having a weight of 50 pounds or more predicated upon a ream having 500 sheets of 25 by 38" sheet size. Common heavy basis weights are 50, 60, 70 and 80 pounds. Common light basis weight webs are 32, 35, 38 and 43 pounds.

Also as noted hereinbefore, conventional coating formulations are useful in the practise of the invention. The following specific examples are illustrative of such compositions and are understood to be not limiting of the invention:

Composition A Coating clay 100 parts by weight.

Pearl starch (enzyme converted) 18 parts by weight, Sodium hexametaphosphate 03 part by weight,

on n 1 part by weight, Water 120 parts by weight ap proximately, Caustic (or ammonia) To adjust to pH of 9.0. Coating solids Approximately 50% by weight.

Composltzon B Coating clay 80 parts by weight, Calcium carbonate 20 parts by weight. Dextrinized starch 16 parts by weight. Sodium hexametaphosphate 0.3 part by weight, Soap 1 part by weight. H O 96 parts by weight approximately. Caustic (or ammonia) To adjust to pH of 9.0. Coating solids Approximately 56% by Weight.

Composition C Premium domestic clay 60 parts by weight. Calcium carbonate 40 parts by weight. Ethylnted starch (ethylene oxide derivative of starch) 20 parts by weight, Sodium hexametaphosphate 0.3 part by weight. Calcium stearate 1 part by weight:v H 76 parts by weight approximately. Caustic (or ammonia) To adjust to pH of 9.0. Coating solids Approximately 62% by weight.

In specific application composition A may be applied. utilizing the coating apparatus described, to a 30 pound basis weight, 40% sulphite, 60% groundwood web. The coating composition on roll 13 of the apparatus is in the form of a thin film of the aqueous coating composition but is applied in metered excess such that coating composition will be available for blading off at the blade nip with roll 14. The composition itself tends to follow the paper web irregularities, coating the hills and valleys of the paper web, and the blading operation eifects removal of and smooths the composition from the hills substantially to the web surface (FIG. 3).

Rolls 13 and 14 in this specific application are 36 in diameter and are driven respectively at surface speeds of 10 898 and 910 f.p.rn. Roll 24 may have a diameter of 12" and is positioned as illustrated in FIG. 1. The web speed itself is substantially that of the backing roll at the pressure nip between the backing roll and doctor blade, although some slight slippage may occur due to the draw of the takeup equipment.

Using composition A and the web described, approximately a 13 pound pickup may be obtained to provide a 43 pound basis weight finished web.

This total pickup of 13 pounds is applicable when the coating is applied to both sides of the web. In the pass of the equipment for the first side coating, a pickup of approximately 6 /2 pounds is attained; the web is then dried and a pass made through a second center for application of coating to the second side, resulting in a total pickup of about 13 pounds. customarily the web is then dried and supercalendered for the development of op timum properties as a printing paper.

It is to be noted that the coating pickup is substantially independent of viscosity, applicable viscosities covering a wide range as noted hereinbefore. However, in the specific example a viscosity of 45 poises is exemplary and efiective.

It is to be understood that the invention is not to be limited to the specific constructions, arrangements, devices and methods shown and described, except only insofar as the claims may be so limited, as it will be apparent to those skilled in the art that changes may be made without departing from the principles of the invention. In particular, it is to be understood that, although certain dimensions, arcs, and angles have been mentioned above, such as the angles a and b and A, and the distances c, d and 2, these have been given for purposes of example only, for better understanding of the invention, and that r the invention is not to be limited to these specific quantities unless such limitations are specifically stated in the claims.

What is claimed is:

i. In apparatus for coating a continuous web of paper, the combination of a rotatable backing roll and a rotatable coating applicator roll defining a first pressure nip therebetween, a doctor blade defining a second pressure nip with said backing roll, said pressure nips bounding a peripheral surface portion of said backing roll, means for moving a paper web through said first pressure nip and said second pressure nip sequentially, and a webearrying rotatable nip-out roll between said pressure nips in the direction of web movement from said first pressure nip to the second pressure nip and positioned to prevent substantial peripheral contact of said web with said surface portion of said backing roll.

2. In paper coating apparatus wherein a paper web is moved from a first pressure nip between a rotatable backing roll and an applicator roll to a second pressure nip between a doctor blade and the said backing roll, the improvement which comprises a rotatable web-carrying nip-out roll between the first pressure nip and the second pressure nip in the direction of web movement from the first pressure nip to receive the web, and means for drawing the web from the said web-carrying nip-out roll through the said second pressure nip, said means and nipout roll being so arranged that a said web has a minimum but positive arc of contact with the backing roll immediately preceding and immediately succeeding said second pressure nip.

3. The method of coating a web of paper which comprises the steps of driving in rotation a coating applicator roll and a backing roll which form a first pressure nip therebetween, passing the web through the said first pressure nip while applying an excess of an aqueous mineral coating composition to the web at the nip, subsequently blading off excess of the mineral coating composition at a second pressure nip formed by said backing roll and a doctor blade, and passing the said web from said first pressure nip to said second pressure nip in a winding path outwardly of the periphery of the backing roll between the said pressure nips while supporting the web to minimize contact of the web with the backing roll between the said nips.

4. The method of coating a Web of paper which comprises the steps of driving in rotation a coating applicator roll and a backing rool which form a first pressure nip therebetween, passing the Web through the said first pressure nip while applying a metered excess of an aqueous mineral coating composition to the said web as the nip, subsequently blading off excess of coating composition from the web surface at a second pressure nip formed by said backing roll and a doctor blade while supporting the web with the backing roll on either side of the blade, passing the said web from said first pressure nip to said second pressure nip in a winding path outwardly of the periphery of the backing roll between the said pressure nips while supporting the web to minimize contact of the Web with the backing roll between the said nips, and drawing the supported bladed web from the backing roll closely adjacent the doctor blade and under tension such that web slippage on the bacning roll is induced.

5. In coating apparatus for webs of paper sheet matetied, the combination of a cotting applicator roll and a backing roll having a nip between them adapted to pass a web of sheet material therethrough, a blade having a nip with said backing roll for passing the sheet material web therethrough to blade off excess coating from the sheet material and smooth the coating on the sheet material, and a nip-out roll located opposite the peripheral surface of said backing roll between said two nips for holding the web out of contact with the peripheral surface of the backing roll intermediate said nips, said nipout roll having a peripheral web carrying portion located between the peripheral surface of the applicator roll and a tangent to said rolls at their nip so that the web passing out of said nip between said rolls passes over a limited peripheral surface of said applicator roll.

6. In coating apparatus for webs of paper sheet material, the combination of a coating applicator roll and a backing roll having a nip between them adapted to pass a web of sheet material therethrough, a blade having a nip with said backing roll for passing the sheet material web therethrough to blade olf excess coating from the sheet material and smooth the coating on the sheet material, a nip-out roll located opposite the peripheral surface of said backing roll between said two nips for holding the web out of contact with the peripheral surface of said backing roll between the two nips, and a take otf roll adapted to receive the web coming from said backing roll and from said blade, said nip-out roll having a web carrying peripheral surface located between the tangent of said backing and applicator rolls at their said nip and the peripheral surface of said applicator roll so that the web passing out of said nip between said rolls passes over a limited peripheral surface of said applicator roll, said nip-out roll and said take up roll having web carrying peripheral surfaces lying substantially on a tangent to said backing roll under said blade for causing the web to pass substantially along the tangent with a minimum amount of contact with the peripheral surface of said backing roll in the vicinity of said blade.

7. In coating apparatus for webs of paper sheet material, the combination of a coating applicator roll and a backing roll having a nip between them adapted to pass a web of sheet material therethrough, a blade having a nip with said backing roll for passing the sheet material web therethrough to blade otf excess coating from the sheet material and smooth the coating on the sheet material, a nip-out roll located opposite the peripheral surface of said backing roll between said two nips for holding the web out of contact with the peripheral surface of the backing roll intermediate said nips, said nip-out roll having a peripheral surface between the tangent to said backing and applicator rolls at their said nip and the peripheral surface of said applicator roll so that the web is drawn across the peripheral surface of the applicator roll in leaving its nip with said backing roll, and means for driving said applicator roll at a slower peripheral speed than said backing roll and at a slower speed than said web for facilitating withdrawal of the web from said applicator roll subsequent to its nip with said backing roll.

8. In coating apparatus for webs of paper sheet material, in combination with a coating applicator roll and a backing roll having a nip between them adapted to pass a web of sheet material therethrough, a blade having a nip with said backing roll for passing the sheet material web therethrough to blade off excess coating and smooth the coating, said blade being located on the periphery of the backing roll at more than degrees and less than degrees around the backing roll periphery from the nip between said rolls in the direction of web travel, and a nip-out roll located opposite the peripheral surface of said backing roll between said nips for holding the web out of contact with the peripheral surface of the backing roll intermediate said nips as the web passes from said first named nip into second named nip.

References Cited in the file of this patent UNITED STATES PATENTS 2,257,373 Fanselow Sept. 30, 1941 2,279,553 Bradt Apr. 14, 1942 2,312,927 Murray Mar. 2, 1943 2,329,034 Buck et a1 Sept. 7, 1943 2,352,658 Richmond et al July 4, 1944 FOREIGN PATENTS 195,883 Germany Feb. 25, 1908 

3. THE METHOD OF COATING A WEB OF PAPER WHICH COMPRISES THE STEPS OF DRIVING IN ROTATION A COATING APPLICATOR ROLL AND A BACKING ROOL WHICH FORM A FIRST PRESSURE NIP THEREBETWEEN, PASSING THE WEB THROUGH THE SAID FIRST PRESSURE NIP WHILE APPLYING AN EXCESS OF AN AQUEOUS MINERAL COATING COMPOSITION TO THE WEB AT THE NIP, SUBSEQUENTLY BLADING OFF EXCESS OF THE MINERAL COATING COMPOSITION AT A SECOND PRESSURE NIP FORMED BY SAID BACKING ROLL AND A DOCTOR BLADE, AND PASSING THE SAID WEB FROM SAID FIRST PRESSURE NIP TO SAID SECOND PRESSURE NIP IN A WINDING PATH OUTWARDLY OF THE PERIPERHY OF THE BACKING ROLL BETWEEN THE SAID PRESSURE NIPS WHILE SUPPORTING THE WEB TO MINIMIZE CONTACT OF THE WEB WITH THE BACKING ROLL BETWEEN THE SAID NIPS. 